Circuit breakers provide automatic current interruption to a monitored circuit when undersired overcurrent conditions occur. These overcurrent conditions include, for example, arc faults, overloads, ground faults, and short-circuits. In a thermal magnetic circuit breaker, an overcurrent is detected when the fault current generates sufficient heat in a strip composed of a resistive element or bimetal to cause it to deflect. The mechanical deflection triggers a trip assembly that includes a spring-biased latch mechanism to force a movable contact attached to a movable blade away from a stationary contact, thereby breaking the circuit. When the circuit is exposed to a current above that level for a predetermined period of time, the trip assembly activates and tripping occurs thereby opening the circuit.
The bimetal deflects in a predictable and repeatable manner across a thermal profile over a period of time, and the rate and extent of deflection is a function of various parameters, including the cross-sectional area (width, thickness), length, and composition of the bimetal element. The bimetal is attached to a yoke that is magnetically coupled to a movable armature. The movement of the bimetal in response to excessive electrical current causes the armature to move relative to the yoke, triggering a chain of mechanical actions that cause the breaker to thermally trip. For magnetic tripping in response to sudden overloads, a magnetic field induced relative to the magnetic yoke causes the armature to be moved relative to the yoke, triggering a magnetic trip.
In miniature circuit breakers, such as the QO® and Homeline® family of circuit breakers available from Square D Company, the width of the bimetal (typically ¼ inch) is limited by the width of the housing (typically ¾ to 1 inch). To decrease the width of the overall miniature circuit breaker, such as in half-size or tandem circuit breakers, the width of the bimetal would have to be decreased as well, but at the expense of the trip ratings for the circuit breaker. Alternately, the thickness of the bimetal would have to be increased in order to maintain the same cross-sectional area, but increasing thickness substantially reduces bimetal flexibility and renders thermal tripping and calibration very difficult if not impossible. Bimetals must maintain a minimum cross-sectional area for a desired I2t (current squared time) capacity in order to be flexible enough to move a given distance when heated. It is desirable to decrease the width of a miniature circuit breaker without encountering these difficulties.
Existing thermal circuit breakers utilize a bimetal that is either received inside a yoke or in line with a yoke. In the former implementations, the width of the bimetal is constrained by the width of the yoke, so a decrease in the width of the circuit breaker results in a reduction in yoke width, which in turn reduces the bimetal width, requiring an increase in its thickness in order to maintain the same I2t value. In the latter implementations, both the width and the length of the bimetal is constrained by the form factor of the circuit breaker. A shorter bimetal is used because it is in line with the yoke. The shorter length reduces the overall effective travel distance of the bimetal so as its width is reduced, its flexibility is significantly reduced by any increase in thickness if the I2t capacity is to be unchanged. It is desirable to decrease the width of a miniature circuit breaker without encountering these difficulties.
Thus, a need exists for an improved apparatus and method. The present invention is directed to satisfying one or more of these needs and to solving other problems.